1. Field of the Invention
The present invention relates to the field of displacement control of plants that respond to one or more bias forces, and in particular to the field of controlling such plants with compensation for those bias forces. More particularly, the present invention has application in bias-compensating control of a head actuator in a magnetic storage disk drive.
2. Description of the Related Art
Many mechanical and electromechanical systems operating with automatic control systems include actuators that respond, or are affected by, one or more bias forces that perturb the actual position or velocity of the actuator away from the position or velocity expected by the control system. In the case of a magnetic storage disk drive, the actuator may be a head actuator for positioning the read-write heads of the disk drive over a desired data track on a rotating disk in the disk drive. Bias forces on such a head actuator may include the gravitational force exerted upon the head and actuator as they are positioned over a track, wind generated by the spinning disk, stress from the cable that usually transfers data between the head and the stationary portion of the disk drive, and so forth.
These bias forces are frequently described as disturbances because they cause the head actuator in the disk drive, or the actuator in the more general system, to respond inaccurately to a control instruction to assume a given position. This inaccuracy necessitates iterative adjustment by the control system to bring the actuator to the desired position, and this adjustment process delays completion of the positioning operation. Thus, modern control systems for plants subject to bias forces desirably compensate for those forces when generating control signals.
Two methods exist for compensating for a disturbance W. The first method involves calculating a compensation value to be injected into the control signal. Calculation of such compensation values requires detailed knowledge of a predictable relationship between the positions the actuator may assume and the disturbances it will experience in those positions. This method has the drawback that because the calculation process takes time, it delays the positioning operation when the target position lies only a short distance from the actuator's current position.
The second and more widely used method calculates disturbance estimation values in advance and stores them in a bias table for later use. This approach also has drawbacks in that it requires an initial calculating process to create the bias table and a separate memory area to store the bias table.
The significance of compensating for bias forces in controlling an actuator has been recognized previously. However, an approach has not heretofore been proposed that provides accurate disturbance compensation with rapid seek response but without requiring substantial storage capacity to store a bias table. For example, U.S. Pat. No. 5,585,976, entitled "Digital Sector Servo Incorporating Repeatable Run Out Tracking" and granted to Pham, discloses digital servo control system for controlling the head of a disk drive. The system compensates for repeatable run out during a seek operation by adding to the control signal a feed forward signal taken from memory. The system uses a plant model to generate predicted values for position, velocity, and bias force. However, in seek mode this system does not take advantage of the differences between long seeks and short seeks in its process for producing predicted bias force values.
Similarly, U.S. Pat. No. 5,404,253, entitled "Estimator-Based Runout Compensation in a Disk Drive" and granted to Painter, shows a disk drive controller that compensates for bias forces; however, the bias force estimation process does not differentiate between long seek operations and short seek operations. U.S. Pat. No. 5,369,345, entitled "Method and Apparatus for Adaptive Control" and granted to Phan et al., discloses an adaptive control system applicable to a disk drive servo actuator apparatus. This system also generates predicted values for position, velocity, and bias force and calculates a compensated control signal using a predicted value for bias force generated by an estimator algorithm. However, the control signal calculation process does not distinguish between long seeks and short seeks.
U.S. Pat. No. 4,949,201, entitled "Disk Drive Head Position Controller with Static Bias Compensation and Plural Velocity Detectors" and granted to Abed, is incorporated herein by reference and a copy of which is annexed hereto. This patent shows a system for accurately and rapidly positioning the head of a hard disk drive. The disclosed system specifically compensates for bias force values in the generation of control signals. The bias values may be computed ahead and stored in RAM; computed by the controller processor during an initialization phase, using a plant model; or computed by the controller processor as needed during seek maneuvers.
The Abed system distinguishes between high velocity and low velocity modes, with the distinction based on the number of tracks the head must move during a seek operation. However, the control signal generation process does not utilize this distinction in generating bias values. Indeed, this system uses a fill-in gain unit instead of scaling the bias values for the low velocity mode.
Thus, a need has existed for an actuator control system, and particularly such a system that is applicable to magnetic disk storage devices, that accurately compensates for bias forces experienced by the actuator but does not sacrifice either response speed or data storage capacity to do so.